http: fix writableFinished and 'finish' after write errors

[pimterry]

On HTTP OutgoingMessages, only emit 'finish' and set writableFinished once all data has actually been flushed successfully, and make end() callbacks report the correct outcome like stream.Writable: called with null on finish, or with the error that prevented the flush.

This changes visible behaviour in some subtle ways, but I think exclusively in a more correct direction, which matches Writable everywhere else and matches the existing docs. Now, if an HTTP write calls end() but fails to actually flush the data, it will fail to finish, and the end() callback behaviour correctly matches. See the 2 tests that need changing here due to subtle changes in ordering & finish behaviour.

It's on the line, but I'd really rather not make this semver major because it's a prerequisite for #64566 fixing #64272. That's a regression on v24 & v26 today, due to changes in how libuv exposes ECONNRESET. To work around that change, we need an accurate signal for writableFinished, and we currently don't have one.

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[codecov]

Codecov Report

❌ Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.16%. Comparing base (396aad0) to head (ddbd846).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/_http_outgoing.js	98.03%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #64847      +/-   ##
==========================================
- Coverage   90.17%   90.16%   -0.02%     
==========================================
  Files         746      746              
  Lines      242776   242814      +38     
  Branches    45741    45755      +14     
==========================================
+ Hits       218929   218930       +1     
- Misses      15347    15375      +28     
- Partials     8500     8509       +9     

Files with missing lines	Coverage Δ
lib/_http_outgoing.js	97.77% <98.03%> (+0.14%)	⬆️

... and 27 files with indirect coverage changes

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